British police have smashed an "audacious" and sophisticated plot to take control of the bank Santander UK's computer systems and steal millions of pounds.

Detectives, who described the plot as the most significant case of its kind, said they had arrested 12 men who had allegedly fitted a device in a Santander branch in southeast London capable of transmitting the entire desktop contents of the bank computer.

"In effect, this allowed the suspects to take control of the bank computers remotely," said a spokesman for London's Metropolitan Police.

"The device had been fitted and officers believed that the group would imminently have been in a position to go operational," he said, adding they had avoided multi-million-pound losses.

Santander UK said a bogus maintenance engineer pretending to be from a telecoms firm had fixed the device, which police described as a keyboard video mouse, to a computer in the Surrey Quays branch.

It said it had been aware of the alleged fraud and had worked closely with officers, adding no staff members had been involved.

"It failed and no money was ever at risk," the bank said. "Santander operates multiple levels of controls to protect customers' funds and this attack would not have been successful."

LONG-TERM OPERATION

Detectives from the Metropolitan Police's Central e-Crime Unit said they had arrested 11 men at an address in west London and another man in central London on Thursday, while houses across the capital were raided.

The 12 suspects, aged between 23 and 50, are being held in custody at a London police station.

Cyber crime has become a major threat to companies and financial institutions. In July, US federal prosecutors charged five men responsible for a hacking and credit card fraud spree that cost companies more than US$300 million, the biggest cyber crime case filed in US history.

A week later, a panel of lawmakers said that Britain was losing the battle against cyber crime and needed a new crack crime unit to fight the growing problem.

The cost of cyber crime to the British economy is estimated to be between £18 billion and £27 billion and British financial services firms lost a total of £5.4 billion as a result of fraud last year, according to government data.

"This was a sophisticated plot that could have led to the loss of a very large amount of money from the bank, and is the most significant case of this kind that we have come across," said Detective Inspector Mark Raymond.